Printed wiring board (PWB) laminates are typically composed of an electronic grade woven glass impregnated with a resin system. If the laminate is fully cured, it is called “C-stage,” while partially cured laminates are known as “B-stage” or “pre-preg.” A PWB is typically made by alternating layers of B-stage and C-stage laminate.
Originally, the resin systems were designed to withstand three major thermal excursions including the bonding of copper foil to the resin impregnated glass cloth, lamination of multiple layers, and the soldering of components to the PWB. Assembly of PWB designs today often require multiple thermal exposures through lamination, solder reflow, and rework/repair. In all, it is not atypical for a laminate to experience numerous high temperature thermal excursions. The effects of these thermal exposures are cumulative and traditional resin systems as well as current resin systems experience thermal breakdowns.
In addition to the increased number of thermal exposures, the use of lead-free solder has lead to increased thermal breakdowns. First, the solder reflow temperatures associated with lead-free solder are approximately forty degrees Celsius higher than traditional tin-lead solder reflow temperatures. Secondly, lead-free solder joints are stiffer than tin-lead solder joints. These two factors, along with the increased number of thermal exposures, transfer more strain into the PWB structure, which in turn, leads to resin shrinkage and fracture (e.g. tail cracking, eyebrowing, delamination, pad cratering).
These problems mostly occur in complex designs; however, even a single lamination is susceptible. The problem of resin fracture seems to occur more frequently with bismaleimide triazine (BT) than glass epoxy resin. As use of BT boards evolved, it became common to build PWB made from glass epoxy inner layers and BT outer layers. This mixed dielectric was lower cost but some applications still require a pure or high percentage BT content. New PWB designs today frequently employ new resin systems. Sometimes these new materials exhibit fracture. Additionally, today's handheld electronic devices are constantly subjected to high stresses and strains which are thermally or mechanically induced shocks occurring during use in their end-use environment. These stresses which can be caused by dropping the devices, sometimes causes the enclosed printed wiring assembly (electrical components soldered on to a PWB) to exhibit pad crater failures. The component, solder attached to the PWB copper pad, mechanically breaks in the PWB or away from the PWB due cracking of the resin under the PWB copper pad resulting in catastrophic electrical failure.
It is therefore an object of the present invention to provide a laminate which will be less susceptible to failures and latent damage including tail cracking, eyebrowing, and pad cratering.
It is further an object of the present invention to provide a printed wiring board which will be less susceptible to failure and latent damage caused by thermal exposure and lead-free solder.
It is another object of the present invention to provide a method for making a laminate which will be less susceptible to failure and latent damage caused by thermal exposure and lead-free solder.